U.S. patent application number 10/791213 was filed with the patent office on 2004-12-02 for applying digital watermarks using printing process correction.
Invention is credited to Reed, Alastair M..
Application Number | 20040240704 10/791213 |
Document ID | / |
Family ID | 24208062 |
Filed Date | 2004-12-02 |
United States Patent
Application |
20040240704 |
Kind Code |
A1 |
Reed, Alastair M. |
December 2, 2004 |
Applying digital watermarks using printing process correction
Abstract
Image color values are modified in accordance with printing
process characteristics. Digital watermark signal representations
are determined and modified in accordance with the characteristics.
The modified signal representations are combined with the original
image color values. The image is then printed by the printing
process. The resulting printed image includes a watermark that is
not materially affected by the printing process
characteristics.
Inventors: |
Reed, Alastair M.; (Lake
Oswego, OR) |
Correspondence
Address: |
DIGIMARC CORPORATION
9405 SW GEMINI DRIVE
BEAVERTON
OR
97008
US
|
Family ID: |
24208062 |
Appl. No.: |
10/791213 |
Filed: |
March 1, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10791213 |
Mar 1, 2004 |
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10209053 |
Jul 30, 2002 |
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6700995 |
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10209053 |
Jul 30, 2002 |
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09553084 |
Apr 19, 2000 |
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6590996 |
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Current U.S.
Class: |
382/100 ;
382/162 |
Current CPC
Class: |
H04N 1/32251 20130101;
G06T 1/0021 20130101; H04N 1/32208 20130101; G06T 1/0028 20130101;
H04N 19/467 20141101; H04H 20/31 20130101; H04N 7/163 20130101;
H04N 21/23892 20130101; H04N 2201/3284 20130101; H04N 1/32309
20130101; H04N 1/32352 20130101; H04N 21/4627 20130101; H04N
21/8358 20130101; G06K 9/00 20130101; H04N 2201/3233 20130101; H04N
1/32203 20130101; G06T 2201/0052 20130101; H04N 1/32154 20130101;
H04N 1/32229 20130101; H04N 1/32288 20130101; H04N 19/645 20141101;
H04N 2201/327 20130101; G06T 2201/0051 20130101; H04N 1/32293
20130101; G06T 1/0064 20130101; H04N 21/44008 20130101 |
Class at
Publication: |
382/100 ;
382/162 |
International
Class: |
G06K 009/00 |
Claims
What is claimed is:
1. A method of digital watermarking an image comprising: adjusting
the image in accordance with values in a first representation
associated with a printing process; determining values to convey a
digital watermark in the adjusted image; adjusting the values in
accordance with a second representation associated with the
printing process; and combining the adjusted change values and the
image to produce a digital watermarked image.
2. The method of claim 1, wherein the first representation
comprises a forward dot gain curve.
3. The method of claim 2, wherein the second representation
comprises a backward dot gain curve.
4. The method of claim 3 wherein the backward dot gain curve
comprises an inverse of the forward dot gain curve.
5. The method of claim 1 wherein the printing process comprises an
offset printing press.
6. The method of claim 1 wherein the image is watermarked using a
scale to black technique.
7. The method of claim 1 wherein said image is watermarked using a
scale to white technique.
8. A method of steganographically hiding a signal in an image
comprising: determining change values to represent the signal in
the image; and altering color values of the image by an amount to
achieve the change values, wherein the amount includes a
compensation for a variation in a relationship of an input color
value and at least one of ink and dye provided by a printing
process to represent the input color value, and wherein the image
includes the signal steganographically embedded therein when
printed with the printing process.
9. The method of claim 8, wherein the printing process comprises an
offset printing process.
10. The method of claim 8, wherein the steganographically hiding
comprises digital watermarking.
11. The method of claim 8, further comprising printing the image,
wherein the printed image includes the signal steganographically
embedded therein.
12. A method of processing an image to compensate for variation in
a printing process, wherein the image includes a plurality of color
values, said method comprising: receiving a representation of a
variation in a relationship of an input color value and at least
one of ink and dye provided by the printing process to represent
the input color value; determining change values needed to alter
the image to accommodate a digital watermark embedded therein;
adjusting the change values with the representation; and modifying
the image with the adjusted change values to accommodate the
digital watermark and to compensate for the variation.
13. The method of claim 12 wherein the printing process comprises
an offset printing press.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/209,053, filed Jul. 30, 2002 (now U.S. Pat.
No. 6,700,995). The Ser. No. 10/209,053 application is a
continuation in part of co-pending U.S. patent application Ser. No.
09/553,084, filed Apr. 19, 2000 (now U.S. Pat. No. 6,590,996). Each
of these patent documents is herein incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates steganography and more
particularly to the digital watermarks.
BACKGROUND AND SUMMARY OF THE INVENTION
[0003] The technology for applying digital watermarks to images and
to other types of data is well developed. For example see issued
U.S. Pat. No. 5,748,783, issued U.S. Pat. No. 5,768,426 issued U.S.
Pat. No. 5,822,435 and the references cited in these patents. Also
various commercially available products (such as the widely used
image editing program Photoshop.TM. marketed by Adobe Corporation)
have image watermarking capability. There are many other patents
and much technical literature available relating to the application
of digital watermarks to images and to other types of data.
[0004] Co-pending application Ser. No. 09/553,084 (now U.S. Pat.
No. 6,590,996) describes a technique of color adaptive
watermarking. With the technique described in application Ser. No.
09/553,084 a change in an image attribute such as luminance (or
chrominance) is mapped to a change in color components such that
the change is less visible application Ser. No. 09/553,084
describes the "scale to black" and the "scale to white" techniques
for applying watermarks. By using the scale to white method for
colors with a high yellow content such as yellow, red and green,
and by using the scale to black for blue, cyan and magenta a
watermark with a lower visibility and the same detect ability can
be embedded in an image.
[0005] It is known that when an image is printed on a standard
offset press, the relationship between the digital value of a color
and the amount of ink actually applied by the press is not linear.
FIGS. 1 illustrates the dot gain curve for a typical standard
offset printing press. The horizontal axis gives a digital value of
a color and the vertical axis indicates the amount of ink actually
transferred by the press. The shape of the dot gain curve of offset
printing presses is well known.
[0006] As a result of the dot gain curve illustrated in FIG. 1,
when an image containing a watermark is printed on an offset press,
a watermark signal in the shadows (i.e. in an area with more ink)
is reduced and a watermark signal in the highlights (i.e. in an
area with less ink) is amplified. Note that the slope of the dot
gain curve is different in the shadow area and in the highlight
area. Thus, the same amount of change in color value produces a
different amount of change in the ink applied in the two different
areas. The present invention provides a technique which insures
that a watermark signal is preserved in an printed image as
accurately as possible not withstanding the fact that the dot gain
curve of the printing press is not linear.
[0007] With the present invention, the image data is first modified
in accordance with the forward dot gain curve of a printing press,
next the watermark "tweak" values (i.e. the watermark change
values) are calculated for this modified image data. The calculated
"tweak" values are then modified in accordance with the backward
dot gain curve of the printing press. The modified tweak values are
then added to the original image data values to produce a
watermarked image. The watermark image is then printed on the
printing press. The result is that the "effective" tweak on printed
paper is not materially affected by the dot gain curve of the
printing press.
BRIEF DESCRIPTION OF FIGURES
[0008] FIG. 1A shows a forward dot gain curve.
[0009] FIG. 1B shows a backward dot gain curve.
[0010] FIG. 2 illustrates scaling to black.
[0011] FIG. 3 illustrates scaling to white.
[0012] FIG. 4 is a program block flow diagram of the operation of
the preferred embodiment.
DETAILED DESCRIPTION OF EMBODIMENTS
[0013] Co-pending application Ser. No. 09/553,084, filed Apr. 19,
2000 (Now U.S. Pat. No. 6,590,996) describes a system for
watermarking images. The system described in application Ser. No.
09/553,084 inserts watermarks in images by selecting and modifying
colors to obtain approximately equal visibility for all colors. The
preferred embodiment of present invention, as described herein, is
described as a modification of the system described in application
Ser. No. 09/553,084. The object of the modifications is to
compensate for the dot gain curve of a printer. The entire
specification of application Ser. No. 09/553,084 is hereby
incorporated herein by reference.
[0014] It is desirable that a watermark embedding algorithm produce
luminance changes with approximately equal visibility through color
space. Adaptive color embedding as described in application Ser.
No. 09/553,084, selects the colors that are modified to produce a
required luminance change, in a way that obtain approximately equal
visibility for all colors. The dot gain correction provided by the
preferred embodiment described herein approximately compensates for
the non-linear effect of the printing process, so that a desired
percentage change is achieved on press (that is, in the amount of
ink applied to create the image). It is noted that the slope of the
dot gain curve is different in the shadow area and in the highlight
area. Thus, the same amount of change in color value produces a
different amount of change in the ink applied in the two different
areas. The preferred embodiment insures that a watermark signal
(i.e. a change value) is preserved in a printed image as accurately
as possible not withstanding the fact that the dot gain curve of
the printing press is not linear.
[0015] As explained in application Ser. No. 09/553,084 a watermark
can be applied to images using either a scale to black or a using a
scale to white technique. With the scale to black technique, the
image pixel is like a vector between black and the pixel color
value. The vector is increased or decreased as shown in FIG. 2.
That is, FIG. 2 illustrates the color changes for a luminance
change utilizing the scale to black technique. The following table
lists for each color, the colors that are modified as a result of a
luminance change. The table also indicates the degree to which the
modification is visible.
[0016] For Scale to Black:
1 Color Colors Modified Visibility of the change yellow
cyan/magenta high red cyan high green magenta medium Blue Yellow
low Cyan Magenta/yellow low Magenta Cyan/yellow low
[0017] FIG. 3 illustrates the color changes that occur with a scale
to white technique. The scale to white technique obtains the same
luminance change as the scale to black technique; however, when
scaling to white the image pixel is a vector between white and the
pixel color value as shown in FIG. 2. The following table lists for
each color, the colors modified as the result of a luminance
change. The table also indicates the degree to which the
modification is visible.
[0018] For Scale to White
2 Color Colors Modified Visibility of change yellow yellow low red
magenta/yellow low green cyan/yellow medium Blue Cyan/magenta high
Cyan Cyan high Magenta Magenta medium
[0019] By using the scale to white method for colors with high
yellow content such as yellow and red, and scale to black for blue,
cyan, magenta and green a lower visibility mark can be made with
the same detectability. Scaling to white results in the watermark
being applied mainly to the dominant colors, and scaling to black
implies that the watermark is mainly in the secondary colors.
[0020] When images are printed on an offset press, it is known that
there is not a straight line relationship between the digital value
of the color at any point in the image and the corresponding amount
of ink applied to the paper at that point. This is known as dot
gain. FIG. 1A shows the forward dot gain curve. That is the
relationship between the digital value of a color and the amount of
ink actually applied. FIG. 2B shows a backward dot gain curve. That
is, FIG. 2 indicates the value needed in order to get a particular
amount of ink on the paper.
[0021] The following is a list of 256 values that generate a curve
as shown in FIGS. 1A. That is, the following is a list of 256
positions on the vertical axis for 256 positions (i.e. for 0 to
255) on the horizontal axis.
3 0 7 12 18 22 26 29 32 34 37 39 42 44 46 48 50 52 54 55 57 59 60
62 64 65 67 68 70 71 73 74 76 77 78 80 81 83 84 85 86 88 89 90 91
93 94 95 96 97 99 100 101 102 103 104 105 106 108 109 110 111 112
113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129
130 131 132 133 134 135 135 136 137 138 139 140 141 142 143 144 144
145 146 147 148 149 150 150 151 152 153 154 155 155 156 157 158 159
160 160 161 162 163 164 164 165 166 167 168 168 169 170 171 171 172
173 174 175 175 176 177 178 178 179 180 181 181 182 183 184 184 185
186 186 187 188 189 189 190 191 191 192 193 194 194 195 196 196 197
198 198 199 200 201 201 202 203 203 204 205 205 206 207 207 208 209
209 210 211 211 212 213 213 214 215 215 216 216 217 218 218 219 220
220 221 222 222 223 224 224 225 225 226 227 227 228 229 229 230 230
231 232 232 233 234 234 235 235 236 237 237 238 238 239 240 240 241
241 242 243 243 244 244 245 246 246 247 247 248 249 249 250 250 251
251 252 253 253 254 254 255
[0022] The following is a list of 256 values that generate the
curve shown in FIG. 1B. That is, the following are the vertical
values for 256 positions (i.e. 0 to 255) on the horizontal
axis.
4 0 1 1 1 1 1 1 1 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4 5 5 5 5 6 6 6 7 7 7
8 8 9 9 9 10 10 11 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 19
19 20 20 21 22 22 23 23 24 25 25 26 27 27 28 29 29 30 31 31 32 33
34 34 35 36 36 37 38 39 40 40 41 42 43 44 44 45 46 47 48 49 49 50
51 52 53 54 55 56 57 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
72 73 74 75 76 77 78 79 80 81 82 83 84 86 87 88 89 90 91 92 93 94
96 97 98 99 100 101 103 104 105 106 107 109 110 111 112 113 115 116
117 118 120 121 122 123 125 126 127 129 130 131 132 134 135 136 138
139 140 142 143 144 146 147 149 150 151 153 154 156 157 158 160 161
163 164 166 167 168 170 171 173 174 176 177 179 180 182 183 185 186
188 189 191 193 194 196 197 199 200 202 203 205 207 208 210 211 213
215 216 218 219 221 223 224 226 228 229 231 233 234 236 238 239 241
243 244 246 248 250 251 253 255
[0023] It is noted that different offset processes produce
different amounts of dot gain; however, with most offset processes,
the dot gain curve has the shape shown. For some particular offset
processes, the actual values may to 50 or 75 percent of the values
given above. The values used in any particular application should
be the values appropriate for the particular printing process that
will be used to print a particular image.
[0024] FIG. 4 is a block program flow diagram of a program for the
preferred embodiment of the invention. The process begins with an
image 401 which is in the CYMK color space. As indicated by block
402, the values for each color in the image are first modified in
accordance with the values of the forward dot gain curve. This
generates a modified image.
[0025] Next as indicated by block 403 calculations are made using
the modified image to determine the "tweak" (i.e. the change)
values needed to embed a particular watermark in the modified
image. This calculation can be done using known watermarking
techniques. In the preferred embodiment, the tweak values are
calculated using the technique available in the commercially
available Photoshop image editing program. However, in other
embodiments, other watermarking techniques can be used.
[0026] The tweak values are next modified in accordance with the
backward dot gain curve values as indicated by block 404. Next as
indicated by block 405, the modified tweak values are added to the
values in the original image 401, thereby producing a watermarked
image. Finally as indicated by block 406 the watermarked image is
printed using an offset press which has the forward and backward
dot gain values used in blocks 402 and 404.
[0027] The watermark can then be read from the printed image using
known watermarks reading techniques.
[0028] In an alternate embodiment of the invention, the tweak
values are added to the modified image values and then the
resultant image is modified in accordance with the backward dot
gain curve values; however, it has been found that in most
instances, the process described in FIG. 4 eliminates some rounding
errors.
[0029] In some applications, it has been found desirable to add
back a constant that controls the amount of the scale to black
signal when a color with high yellow-blue saturation is being
embedded. This is sometime necessary, since some cameras are
insensitive in the blue channel, so changes in yellow are not
detected very well.
[0030] In general to dot gain correction is only applied to the CMY
channels, and not to K channel. However, if desired the dot gain
correction can be applied to all the channels.
[0031] The preferred embodiments described above relate to the dot
gain curve for offset printing processes. It is noted that other
processes such as ink jet printing have a different type of dot
gain curve. The invention can be applied to most types of printing
processes by merely using a dot gain curve appropriate to the
particular process.
[0032] Images watermarked using the embodiments described above can
be read with conventional watermark reading techniques. Naturally
as is conventional the watermark reading technique used should
coincide with the particular technique used to generate the change
values, that is, with the technique used to watermark the
image.
[0033] While the invention has been described with respect to
watermarking images it should be understood that the principle is
applicable to other types of data.
[0034] The preferred embodiment relates to an image in the CYMK
color space. Other embodiments using the same principles can
operate on images in various other color spaces.
[0035] While the invention has been shown and described with
respect to preferred embodiments, it should be understood that
various changes in form and detail may be make without departing
from the spirit and scope to the invention. The scope of the
invention is limited only by the appended claims.
* * * * *